Anatomy & Physiology Chapter 12 PDF

Chapter 12 Nervous System: Nervous Tissue *See separate Image PowerPoint slides for all...

Chapter 12 Nervous System: Nervous Tissue *See separate Image PowerPoint slides for all figures and tables pre-inserted into PowerPoint without notes. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1 General Functions of the Nervous System This communication and control system has three functions 1. Collect Information Sensory receptors detect stimuli to gather information 2. Processes and evaluates information Integrative neurons use sensory information to create sensations, memory, thoughts, and make decisions 3. Initiate response to information Decisions are acted on by motor neurons connected to effectors 2  Organization of the Nervous System Structural organization: Central Nervous System (CNS) Brain Spinal cord Peripheral Nervous System (PNS) Cranial nerves Spinal nerves 3  Organization of the Nervous System Functional organization: – Sensory (afferent) division Receives sensory information from receptors and transmits it to CNS Somatic sensory system detects stimuli we consciously perceive Visceral sensory system detects stimuli we typically do not perceive – Motor (efferent) division Transmits impulses from CNS to effector organs – Somatic nervous system » Conducts voluntary impulses from CNS to skeletal muscle – Autonomic nervous system (ANS) » Conducts involuntary commands to visceral motor nerve fibers » Has Sympathetic and Parasympathetic subdivisions Functional Organization of the Nervous System 5 Nervous Tissue Nervous tissue cells make up the bulk of the nervous system Neurons or nerve cells receive stimuli and transmit action potentials Neuroglia – Support and protect neurons Other tissues help make the functional organs of the nervous system 7  Structure of a Nerve Peripheral nerves may contain the axons of both sensory and motor neurons of the somatic and ANS Nerves have connective tissue wrappings – Epineurium: encloses entire nerve Fascicle Peripheral nerve – Perineurium: wraps fascicle – Endoneurium: wraps an individual axon Motor neuron ending Nerves are vascularized Axon – Blood vessels branch through epineurium and perineurium to become capillaries; allow for exchange between axons and blood Sensory receptor 8 Nerves and Ganglia Structural classification of nerves – Cranial nerves: extend from brain – Spinal nerves: extend from spinal cord Functional classification of nerves – Sensory nerves contain sensory neurons sending signals to CNS – Motor nerves contain motor neurons sending signals from CNS – Mixed nerves contain both sensory and motor neurons 9  Nerves and Ganglia Mixed nerves contain both sensory and motor neurons – Individual axons in these nerves transmit only one type of information A ganglion is a cluster of neuron cell bodies in the PNS 10  General Characteristics of Neurons The neuron is the structural and functional unit of the nervous system Neurons are large specialized cells with these characteristics: – Excitability: responsiveness to a stimulus Stimulus causes change in cell’s membrane potential – Conductivity: ability to propagate electrical signal Voltage-gated channels along membrane open sequentially – Secretion: release of neurotransmitter in response to conductive activity Messenger is released from vesicle to influence target cell – Extreme longevity: cell can live throughout person’s lifetime – Amitotic: After fetal development, mitotic activity is lost in most neurons 11  Neuron Structure Neurons can vary in their size and shape, have a cell body, and one or more processes – Cell body (soma) Plasma membrane encloses the cytoplasm and nucleus Contain Nissl bodies made of ribosomes Integrates graded potentials – Dendrites receive potentials Short unmyelinated processes – Axons conduct action potentials Single long process emanating from cell body at axon hillock Axoplasm and Axolemma specialized for conductivity Synapse with other neurons or effectors Synaptic knobs contain synaptic vesicles with neurotransmitters 12  Neuron Structure Cytoskeleton – Composed of microfilaments, intermediate filaments, microtubules – Provide tensile strength Axons move material to and from the cell body – Anterograde transport: from cell body – Moves newly synthesized material toward synaptic knobs – Retrograde transport: to cell body – Moves used materials from axon for breakdown and recycling in soma 13  Classification of Neurons Structural classification by number of processes coming off soma – Multipolar neurons: many dendrites, one axon – Most common type (99%) – Neurons of CNS including motor neurons – Bipolar neurons: one dendrite and one axon – Specialized sensory neurons – Unipolar neurons (pseudounipolar): one process extends from cell body – Splits into two processes – Most sensory neurons 14  Functional Classification of Neurons Sensory Neurons – Afferent – Carry impulse to CNS – Most are unipolar – Some are bipolar Interneurons – Association – Link neurons – Multipolar – Located in CNS Motor Neurons – Multipolar – Carry impulses away from CNS – Carry impulses to effectors 15  The Synapse Nervous system works because information flows from neuron to neuron Neurons are functionally connected by synapses, junctions that mediate information transfer – From one neuron to another neuron – Or from one neuron to an effector cell Two types of Synapses: – Chemical Synapse – Electrical Synapse  The Chemical Synapse This is where released neurotransmitters cross the synaptic cleft and react with specific molecules called receptors in the postsynaptic neuron membrane. – Depends on release, diffusion, and receptor binding of neurotransmitters – Ensures unidirectional communication between neurons Presynaptic neurons release neurotransmitters Postsynaptic neurons receive signals 17 The Chemical Synapse Synaptic cleft: small fluid-filled gap between the two neurons Events of synaptic communication: Neurotransmitter molecules released from vesicles of synaptic knob into cleft Neurotransmitter diffuses across cleft and binds to postsynaptic receptors Binding of neurotransmitter to receptor initiates postsynaptic potential (a graded potential) Synaptic delay: time it takes for all of these events  Cells of the Nervous System Neuroglia (Glial cells) – Non-exciteable support cells for neurons in Dendrites the CNS and PNS – Differ in size and shape due to different supportive roles Neuroglia of the CNS: – Astrocytes Cell body – Ependymal Nuclei of neuroglia – Oligodendrocytes – Microglia Neuroglia of the PNS: – Schwann cells – Satellite cells Axon 19 © Ed Reschke  Neuroglial Cells of the CNS 1) Astrocytes Processes form feet that cover the Neuron surfaces of neurons and blood vessels and the pia mater. Regulate what substances reach the CNS from the blood Part of Blood Brain Barrier Aid metabolism of certain substances Foot Form structural support for neurons processes Assist neuronal development Astrocyte Induce synapse formation Capillary Occupy the space of dying neurons 20  Neuroglial Cells of the CNS 2) Ependymal cells Ciliated cells Cilia Line ventricles of brain Line central canal of spinal cord With blood vessels form choroid plexus (a) Ependymal Secretes CSF cells Cilia helps circulate Ependymal cells (b) 21  Neuroglial Cells of the CNS 3) Oligodendrocytes Oligodendrocyte Myelinating cell of CNS Single oligodendrocytes can form myelin sheaths around Nodeof Axon portions of several axons. Ranvier Myelin sheath Part of another oligodendrocyte 4) Microglia Wander the CNS and replicate in infection Specialized phagocytic cell microorganisms foreign substances Microglial cell necrotic tissue 22  Neuroglial Cells of the CNS Figure 12.5 Access the text alternative for slide images. 23 Neuroglial Cells of the PNS 1) Satellite cells Surround neuron cell bodies in ganglia Provide insulation, support, and nutrients 2) Neurolemmocytes (Schwann Cells) Produce myelin sheaths found on peripheral neurons Speed up neurotransmission 24  Myelination of Axons Myelin sheath – Composed of myelin, a whitish, protein-lipid substance – Function of myelin Protect and electrically insulate axon Increase speed of nerve impulse transmission – Myelinated fibers: segmented sheath surrounds most long or large-diameter axons – Nonmyelinated fibers: do not contain sheath Conduct impulses more slowly  Myelination of Axons Axons which are tightly wrapped by neuroglial cells are termed myelinated. Cell membranes encircle the axons of neurons and the cytoplasm forming the myelin sheath Neurolemmocyte’s cytoplasm and nucleus are pushed to periphery forming neurilemma – Gaps between the sheathes are nodes of ranvier (neurofibril nodes) 26  Unmyelinated Axons In PNS – Axon sits in depressed portion of neurolemmocyte In CNS – Unmyelinated axons not associated with oligodendrocytes 27 Clinical Application: Multiple Sclerosis  Axon Regeneration After traumatic injuries, PNS axons can regenerate if: Neuron cell body is intact Enough neurilemma remains Steps of axon regeneration – Axon severed by trauma – Proximal to the cut: the axon seals off and swells – Distal to the cut: the axon and sheath degenerate but the neurilemma survives – Neurilemma and endoneurium form a regeneration tube – Axon regenerates guided by nerve growth factors released by neurolemmocytes – Axon reinnervates original effector or sensory receptor 29  Neuron Excitation Neurons are excitable because they maintain a RMP that can change when stimulated Transport proteins in the neurolemma maintain RMP and controlled by different stimuli – Some of them are protein channels controlled by ligands – Some are voltage gated – Sensory neurons have protein channels specific to a stimulus and are found on the dendrites of those sensory neurons  Distribution of Ions Generating a RMP depends primarily on differences in K+ and Na+ concentrations inside and outside the cell and on differences in permeability of the plasma membrane to these ions Potassium (K+) ions are the major intracellular positive ions. Sodium (Na+) ions are the major extracellular positive ions. This distribution is largely created by the Sodium/Potassium Pump, but ion channels in the cell membrane maintain it.  Membrane Channels The role of membrane channels are to establish and control the potential across the membrane – These are membrane proteins that maintain the selective permeability of the cell with the Sodium/Potassium Pump Specific to certain ion – Types of membrane ion channels: Leak (nongated) channels, which are always open Chemically gated channels open when a neurotransmitter binds Voltage-gated channels open at particular charges across the membrane  Membrane Channels Modality gated channels – Normally closed, but open in response to specific type of sensory stimulus For example, change in temp, pressure, light – Found in membranes of sensory neurons that respond to changes in their environment For example, some receptor neurons of the skin have modality gated channels that open in response to mechanical pressure 33  Distribution of Pumps and Channels Functional segments in a neuron: – Receptive segment (dendrite and cell body) Chemically gated channels (For example, chemically gated Cl– channels) – Initial segment (axon hillock) Voltage-gated Na+ channels and voltage-gated K+ channels – Conductive segment (axon and its branches) Voltage-gated Na+ channels and voltage-gated K+ channels – Transmissive segment (synaptic knobs) Voltage-gated Ca2+ channels and Ca2+ pumps 34  Basic Principles of Electricity Voltage: a measure of potential energy generated by separated charge – Measured between two points in volts (V) or millivolts (mV) – Called potential difference or potential Current: flow of electrical charge (ions) between two points – Can be used to do work – Flow is dependent on voltage and resistance Resistance: hindrance to charge flow – Insulator: substance with high electrical resistance – Conductor: substance with low electrical resistance 35  Neurons and Ohm’s Law Neuron activity dependent upon electrical current Ohm’s law – Current = voltage/resistance (I = V/R) – Current increases with larger voltage and smaller resistance As applied to neurons – Charged particles are ions, and current is generated when ions diffuse through channels – Voltage exists across the membrane due to unequal distribution of ions – The membrane offers resistance to ion flow, and this resistance changes due to the actions of gated channels – Resistance decreases when channels open 36  Neuron at Rest Ions are unevenly distributed across the plasma membrane due to the actions of pumps Higher concentration of K+ in cytosol verses interstitial fluid (IF) Higher concentrations of Na+, Cl–, Ca2+ in IF than in cytosol Gated channels are closed in the functional segments of the cell Resting membrane potential (RMP) is typically –70 mV (Higher than Muscle cells) Calcium concentration gradient exists at synaptic knob 37  Neuron at Rest Resting membrane potential (RMP) Negatively charged proteins are part of the cytoplasm that contribute to the negative charge K+ diffusion is the most important factor in maintaining RMP K+ diffuses out of the cell due to its concentration gradient but limited because of negative charge Na+ also influences RMP, but with a few leak channels it keeps the RMP higher than muscle fibers and keeps the Na+/K+ pumps working By pushing 3 positive charges out and pushing in only 2, the pump contributes about –3 mV More importantly, it maintains the concentration gradients for these ions  Physiologic Events in the Neuron Segments Different physiologic events occur at each neuron segments Changes in membrane potential are used as signals to receive, integrate, and send information Membrane potential changes when: – Concentrations of ions across membrane change Membrane permeability to ions changes Changes produce two types of signals – Graded potentials Incoming signals received locally by the soma and dendrites – Action potentials Outgoing signal sent down the axon  Receptive Segment Graded potentials are small, short-lived changes in the RMP that result from gated ion channels opening They are established in the receptive segment by the opening of chemically gated ion channels Magnitude varies from small to large depending on stimulus strength or frequency They can be large or small They can cause a depolarization or hyperpolarization 40  Postsynaptic Potentials Graded potentials in a postsynaptic neuron = postsynaptic potentials – Excitatory PostSynaptic Potential (EPSP) – Neurotransmitter binding opens Na+ channels resulting in local net graded potential depolarization – If it does not reach threshold, but is depolarized, it is facilitated – Action potential of postsynaptic neuron becomes more likely 41  Postsynaptic Potentials Graded potentials in a postsynaptic neuron = postsynaptic potentials – Inhibitory PostSynaptic Potential (IPSP) – Neurotransmitter binding to receptor opens chemically gated channels that allow entrance/exit of ions that cause hyperpolarization Makes postsynaptic membrane more permeable to K+ or Cl– – Action potential of postsynaptic neuron becomes less likely 42 Summation Many neurons receive several EPSP’s and IPSP’s from thousands of other neurons A single graded potential cannot induce an AP, but EPSPs can summate to influence postsynaptic neuron Summation is a total of those inputs and effects the initial segment (axon hillock) when it reaches threshold  Summation If threshold is reached at the initial segment, voltage-gated channels open, and an AP is generated Typically, threshold is about –55 mV Generally, multiple EPSPs must be added to reach threshold Summation occurs across space and time Spatial summation Multiple locations on receptive segment receive graded potentials Temporal summation A single presynaptic neuron repeatedly produces multiple EPSPs within a very short period of time  Initial Segment All-or-none law – If threshold not reached, voltage-gated channels stay closed – If threshold reached, action potential generated and propagated down axon without any loss in intensity – The axon shows same intensity of response to values greater than threshold – Similar to what occurs with a gun Summation of Postsynaptic Potentials - YouTube 45  Conductive Segment The axon (axolemma) conducts action potentials Action potential involves depolarization and repolarization – Depolarization is gain of positive charge as Na+ enters through voltage-gated Na+ channels – Repolarization is return to negative potential as K+ exits through voltage-gated K+ channels Action potential is propagated down axon to synaptic knob – Voltage-gated channels open sequentially down axolemma – Propagation is called a nerve signal or nerve impulse https://www.youtube.com/watch?v=b2ctEsGEpe0 46  Action Potentials Threshold stimulus reached (-55mV) – Voltage gated Na+ channels open – Na+ enters cell and completely depolarizes axoplasm – This opens voltage gated K+ channels – Voltage gated Na+ channels close at maximum depolarization – As Na+ stops diffusing in and K+ continues to diffuse out, repolarization occurs – Below threshold the gated K+ channels close but more slowly – Brief period of hyperpolarization occurs as Na+/K+ pump reestablishes RMP https://www.youtube.com/watch?v=kxnb_TSqmFY  Action Potentials A nerve impulse is the propagation of action potentials down the length of an axon.  Refractory Period Sensitivity of area to further stimulation decreases for a time Absolute Refractory Period – Time when threshold stimulus does not start another action potential Relative Refractory Period – Time when stronger threshold stimulus can start another action potential 50  Action Potential Propagation Propagation of the action potential is caused by voltage gated channels opening sequentially as they are activated Propagation only occurs in one direction as each region of the axon goes through its own potential change – Each region has to repolarize before it can undergo another potential (refractory period) https://www.youtube.com/watch?v=pbg5E9GCNVE 51  Impulse Conduction The speed of impulse conduction varies on different types of neurons. Rate of AP propagation depends on two factors: – Axon diameter Larger-diameter fibers have less resistance to local current flow, so have faster impulse conduction – Degree of myelination Two types of conduction depending on presence or absence of myelin – Continuous conduction – Saltatory conduction 52  Conductive Segment In a non-myelinated axon, voltage can leak across the membrane slowing propagation. Voltage-gated Na+ and K+ channels keep the action potential going along the axon, so AP continues. Continuous conduction is slower because it takes time for ions (and the associated charge) to diffuse and for gates of channel proteins to move  Conductive Segment Myelinated axons transmit impulses through saltatory conduction, which is faster than impulses along unmyelinated axons. – The AP can diffuse faster through the axoplasm of a myelinated segment because the ions can’t leak across the membrane – The propagation isn’t slowed by numerous voltage-gated channels, but fewer ones spaced only in the nodes 54  Conductive Segment The speed of impulse conduction varies on different types of neurons. Nerve fibers are categorized according to their speed of conduction. – Type A: large-diameter, myelinated. Conduct at 15-150 m/s. Motor neurons supplying skeletal and most sensory neurons – Type B: medium-diameter, lightly myelinated. Conduct at 3-15 m/s. Part of ANS – Type C: small-diameter, unmyelinated. Conduct at 2 m/s or less. Part of ANS 11-55  Transmissive Segment Synaptic knob of a chemical synapse – Ensures unidirectional communication between neurons – AP arriving at synaptic knob causes voltage gated Ca2+ channels to open. Ca2+ diffuses into knob – Ca2+ binds to proteins associated with synaptic vesicles and triggers exocytosis Vesicles fuse with membrane, neurotransmitter released – Neurotransmitter binds to ligand Botulinum toxin (e.g., Botox®) messes with this process. receptors on postsynaptic cell 56  Transmissive Segment Neurons can synthesize and release more than one neurotransmitter – Only one type is released at a time so which ones are released depend on the frequency of AP’s at the synaptic knob Effects of neurotransmitters vary. – This depends on the receptors in the postsynaptic cell – Ligand gated ion channels respond to neurotransmitter, creating graded potentials. 57  Potentiation Synaptic potentiation – Repeated use of synapse increases ability of presynaptic cell to excite postsynaptic neuron Ca2+ concentration increases in presynaptic terminal, causing release of more neurotransmitter Leads to more EPSPs in postsynaptic neuron – This can be viewed as a learning process that increases efficiency along a neural pathway – Long-term potentiation: learning and memory  More on Action Potentials If a neuron axon responds at all, it responds completely with an AP The amplitude of an AP can’t get any bigger or longer – Voltage gated channels close at +30mV – AP’s can’t be sustained so… All nerve impulses carried on an axon are the same strength 59 More on Action Potentials Neurons communicate the strength of the stimulus by increasing the number of AP’s The CNS interprets the strength of the stimulus from frequency of potentials  Neurotransmitters Synthesized by neurons and stored in synaptic vesicles Most neurons make two or more neurotransmitters – Usually released at different stimulation frequencies – Neurons can exert several influences this way Over 100 neurotransmitters have been named and are classified by: – Chemical structure – Function  Neurotransmitters Four main chemical classes of neurotransmitters – Acetylcholine (ACh) – Biogenic amines (monoamines) Amino acids with the -COOH group removed and -OH added – (ex.. dopamine and serotonin) – Amino acids – (ex.. Glutamate, glycine, serine, and GABA) – Peptides (Neuropeptides) – (ex.. Enkephalins, beta-endorphins, and substance P) This classification is most useful in discerning receptors which they bind, synaptic delay, and degradation.  Neurotransmitters Neurotransmitters can also be grouped into two functional classifications: – Effects on membrane potential Neurotransmitter effects can be excitatory and/or inhibitory BUT the effect is more determined by receptor to which it binds – Actions on postsynaptic cell Direct action: neurotransmitter binds directly to and opens ion channels Indirect action: neurotransmitter acts through second messenger systems – More diverse effects possible  Neurotransmitters Acetylcholine (ACh) is the best characterized neurotransmitter – Used in PNS to stimulate skeletal muscle; used in the CNS to increase arousal – Effect on target cell depends on receptor present – Nicotinic vs. muscarinic receptors » ACh binds nicotinic receptors to open ion channels and cause EPSP » ACh binds muscarinic receptors to activate second messenger system involving G protein » Can result in EPSP or IPSP 64  Neuromodulation Neuromodulator: chemical messenger released by neuron that does not directly cause EPSPs or IPSPs but instead affects the strength of synaptic transmission – Facilitation – Modulation that causes greater response in postsynaptic neuron – May increase amount of neurotransmitter in cleft or number of postsynaptic receptors – Inhibition – Modulation that causes weaker response – May decrease amount of neurotransmitter in cleft or number of postsynaptic receptors 65  Clinical Applications Drugs can modulate the action of neurotransmitters at the synapse 66  Neuronal Pools Neural integration: neurons functioning together in groups (neuronal pools) to perform a common function – Integrate incoming information received from receptors or other neuronal pool Neuronal pools are also integrated with other pools to receive information and generate output to other neurons and pools 67  Neuronal Pools Neurons within neuronal pools form circuits. – potentiation These patterns of synaptic connections in and between neuronal pools are of four types – Converging – Diverging – Reverberating – Parallel after discharge 68  Neural Integration Four types of circuits are seen in neural integration – Converging Input converges on a single neuron or neuronal pool – Diverging Spreads information from a single neuron to several neurons or neuronal pools – Reverberating Uses feedback to produce repeated cyclical activity – Parallel after discharge Input transmitted simultaneously along several paths 69  Study Questions 1. What is the function of receptors? What are the different types of effectors controlled by the nervous system? 2. Compare and contrast the two primary functional divisions of the nervous system. 3. What are the three connective tissue wrappings in a nerve, and what specific structure does each ensheathe? 4. Explain the neuron characteristics of excitability, conductivity, and secretion. 5. What are the functions of these neuron structures: dendrites, axon, synaptic vesicles, and neurofibrils? 6. How are the different processes that extend from a cell body used to structurally classify neurons? 7. Where are interneurons located, and what is their function? 8. What is a chemical synapse within the nervous system, and how does it function? 9. If a person suffers from meningitis (an inflammation of the meningeal coverings around the brain), which type of glial cell usually replicates in response to the infection? 10. Which specific type of glial cell forms a myelin sheath associated with axons in the PNS? 11. What is the function of the myelin sheath? How does myelination of axons occur in the PNS? 12. Which functional segment of a neuron contains chemically gated channels? Which functional segments contain voltage-gated channels? 13. What is the role of ions and plasma membrane channels in neurons relative to the concepts of current, voltage, and resistance? 70  Study Questions 14. Describe the conditions of a neuron at rest in terms of the RMP; concentration gradients for Na+, K+, Cl– along the entire neuron and Ca2+ at the synaptic knob; and the state of the gated channels. 15. Explain how an RMP is established and maintained in neurons. 16. How are EPSPs and IPSPs established in the receptive segment of a neuron? 17. What is the significance of the threshold membrane potential in the initial segment of a neuron? 18. How do depolarization and repolarization occur in the conductive segment of a neuron? 19. Explain propagation of an action potential – in both an unmyelinated axon and a myelinated axon. 20. What is the sequence of events from the arrival of the propagated action potential (a nerve signal) at the synaptic knob until the release of neurotransmitter into the synaptic cleft? 21. Explain how action potentials differ from graded potentials. 22. What are the general characteristics of group A nerve fibers, and what functions do they normally serve? 23. Explain how frequency of action potentials differs from the velocity of an action potential propagation. 24. Describe how neurotransmitters are classified based upon structure and function. 25. How are neurons arranged in a converging circuit? 26. What are the differences between a reverberating circuit and a parallel-after-discharge circuit? 71